ADP-Ribosylation can be defined as the post synthetic modification of protein by the covalent attachment of the ADP-ribose moiety of nicotinamide-adenosine dinucleotide, abbreviated NAD.sup.+. The ADP-ribosylation of nuclear proteins has recently been reviewed by M. R. Purnell, P. R. Stone, and W. J. D. Whish (Biochem. Soc. Trans., 8, 215, 1980).
The enzyme responsible for ADP-ribosylation is poly (ADP-ribose) synthetase. The ADP-ribose moiety of NAD.sup.+ is cleaved at the nicotinamide-ribose bond and transferred to a protein or a protein-bound ADP-ribose molecule to give a protein bound monomer of ADP-ribose or a polymer of ADP-ribose bound covalently to specific protein. This polymer may be enzymatically degraded by an enzyme poly (ADP-ribose) glycohydrolase, which hydrolyzes the pyrophosphate bonds.
In 1958 it was suggested by R. K. Morton (Nature, 181 540, 1958) that NAD.sup.+ played a key role in the regulation of cellular proliferation. Hasagawa and coworkers (S. Hasagawa, S. Fujimura, Y. Shimizu and T. Sugimura: Biochem. Biophys. Acta., 149, 369, 1967) pointed out that poly (ADP-ribose) synthetase modified nuclear proteins. In 1976 Rechsteiner and coworkers (M. Rechsteiner, D. Hillyard, and B. M. Olivera: Nature, 259, 695, 1976) showed that the cellular half-life of NAD.sup.+ is one hour, which suggests a high turnover of NAD.sup.+ due to the utilization by poly (ADP-ribose) synthetase. Thus, more adenine leaves NAD than enters DNA. Caplan and Rosenberg (A. I. Caplan and M. J. Rosenberg: Proc. Nat'l. Acad. Sci., USA, 72, 1852, 1975) were the first to suggest an involvement of ADP-ribosylation in cellular differentiation which has since been supported by other workers (M. R. Purnell, P. R. Stone, and W. J. D. CWhish: Biochem. Soc. Trans., 8, 215, 1980). Berger and coworkers (N. A. Berger, J. W. Adams, G. W. Sikorski, S. J. Petzold, and W. T. Shearer: J. Clin. Invest., 62, 111, 1978) have shown that chronic lymphatic leukemic cells isolated from patients were higher in poly (ADPR) synthesis than normal cells. It has recently been shown that in Ehrlich ascites cells the histone H1 is ADP ribosylated mainly in the C-terminal fragment (H. C. Braeuer, P. Adamietz, U. Nellessen, and H. Hilz: Eur. J. Biochem., 114, 63, 1981). Kidwell (W. R. Kidwell: J. Biochem., 77, 6, 1975) has postulated that poly ADPR formation acts as a trigger for the cell cycle. NAD.sup.+ levels are known to be lower in malignant cells than in normal cells (L. S. Jedeiken and S. Weinhouse: J. Biol. Chem., 213, 271, 1955). This may be due to the greater utilization of NAD.sup.+ for poly ADPR synthesis in tumor cells.
Novikoff hepatoma cells have been reported to have twice the poly ADPR synthetase of normal liver cells. (L. Burzio and S. S. Koide: FEBS Letters, 20, 29, 1972). Poly ADPR synthetase activity has been shown to be 20 times higher in leukemic lymphoblasts as compared to unstimulated normal lymphocytes (A. R. Lehman, S. Kirk-Bell, S. Shall, and W. J. D. Whish: Exp. Cell Res., 83, 63, 1974). Increased cellular proliferation has clearly been demonstrated to correlate with increased activity of poly ADPR synthetase (For a review see H. Hilz and P. Stone: Rev. Physiol. Biochem. Pharmacol., 76, 1, 1976). Formycin B inhibits cellular proliferation in L-5178Y mouse leukemia cells and it has been postulated by Muller that its cytostatic action is due to inhibition of poly ADPR formation (W. E. G. Muller and R. K. Zahn: Experientia, 31, 1014, 1975).
Thus we hypothesized that certain novel dinucleotides could be prepared which might act as substrate analogs that might bind to poly ADPR synthetase but could not be utilized by this enzyme, thus resulting in an inhibition of ADP-ribosylation and in controlling rapid cellular proliferation, which would have a direct application in the treatment of cancer.
Similarly, certain viral regulated ADP-ribosylation processes necessary for viral propagation might also be selectively inhibited by the novel dinucleotides which by inhibition of viral replication could be useful in the treatment of various viral infections.